Question

How does a student know that a chemical reaction has occurred?

Answer 1

Hint: The chemical change occurs when a substance is changed into a new substance. The initial substance can be one or more and the new substance which is formed also can be one or more. It indicates that the chemical composition of a substance changes. We must know that a chemical substance can neither be created nor destroyed in a reaction, but we can rearrange the substance from one form to another. The new substance can have any state of matter.

Complete step-by-step answer:
Colour change: We have seen rust, right?
We have observed old iron gates have the rust. When the iron metal reacts with oxygen and water in the moisture/ air it forms a compound which is iron oxide (i.e., ferric oxide) called rust. This occurs because of corrosion of the metal. The change in colour from given colour to reddish-brown shows the chemical change i.e., a chemical reaction.
\[4Fe\,\, + \,\,3{O_2}\,\, \to \,\,2F{e_2}{O_3}\]

Temperature change: When there is a change in a chemical reaction, there occurs energy transfer. One of the examples is fireworks, where the chemical change that produces a temperature change and emits the light. Fireworks contain oxidizing agents and fuel. Since the oxidizer breaks down the fuel and releases an enormous amount of energy. Some of the energy is released as heat.

Precipitation formation: When two salts are mixed and form a solid substance in a liquid is called precipitation. This chemical change helps to identify whether the water is good to consume or not. Since this chemical combines with impurity and forms precipitate in water. The impurities can be removed from the water and can be purified.

Note: The chemical change is accompanied by a colour change, or by temperature change, or by the formation of an insoluble precipitate or by the light emission, or by the evolution of a gas. In this, the chemical reaction occurs which changes the state (i.e., physically).
\[C{O_2}(s) \to C{O_2}(g)\]
\[C{H_4}(g) + 2{O_2}(g) \to C{O_2}(g) + 2{H_2}O(l) + \Delta \]